Photovoltaic cells for converting light to electricity have been implemented in products from remote electrical supply systems to calculators. Flat panel displays for computers, televisions and calculators are also widely used. Photovoltaic cells and flat panel displays consist of a semiconductor coated with a transparent conductive metal oxide thin film. In a photovoltaic cell, the semiconductor material is where the light is converted to electricity, and the conductive thin film collects the electricity and conducts it to a buss. The conductive thin film also serves to protect the semiconductor from dust and environmental degradation. In a photovoltaic cell, the coating must be optically transparent to permit light to reach the semiconductor, and in a flat panel display, the coating must be optically clear in order to see the information on the display, hence the adjectives transparent conductive. Because the only known materials to exhibit these properties are metal oxides, and because light transmission is improved with thinner rather than thicker layers of the material, they are fully specified as transparent conductive metal oxide thin film.
These films are presently made by several process, for example sputtering, spray pyrolysis, chemical vapor deposition, and dip coating. In order to have the desired properties of transparency and conductivity, the sputtered or deposited material must have from about 1 to about 2 atomic percent of the metal in the metal oxide in a reduced state. Outside that range, either the transparency, conductivity or both suffer. It is very difficult to control the sputtering and chemical vapor deposition processes in terms of the amount of material that is reduced. Thus, the resulting metal oxide coating must be tested to identify those with the desired characteristics of transparency and conductiivity.
Oxides that have been found to be most useful as transparent conductive metal oxide thin films are tin oxide (SnO.sub.2), indium tin oxide (InSnO.sub.2) and zinc oxide (ZnO).
A paper ELECTROCHEMICALLY REDUCED POLYCRYSTALLINE TIN OXIDE THIN FILMS, H.Feng, S. J. Laverty, P. Magure, J. Molloy, and B. J. Meenan, J. Electrochem. Soc. Vol. 143, No. 6, June 1996, reports conductivity enhancement in polycrystalline tin oxide thin films by electroplating copper onto sidewalls of the polycrystalline tin oxide thin films.
U.S. Pat. No. 4,000,346 to Dowell discusses optically transparent electrically conducting coatings of noble metal oxides. Dowell's films are made by brushing a noble metal salt solution onto an optically transparent substrate, followed by heating from about 350.degree. C. to about 700.degree. C. in an inert atmosphere to form the conductive layer. Disadvantages of this invention include, the high temperature necessary for formation of the conductive layer, and the need for an inert atmosphere.
U.S. Pat. No. 5,078,803 to Pier et al. discusses solar cells (photovoltaic cells) with hazy zinc oxide as contrasted with optically clear zinc oxide and reports an optimum balance of optical and electrical properties for photovoltaic devices. The haziness is achieved either by variation in formation parameters (e.g. chemical vapor deposition) and/or by post formation treatment. Variation of formation parameters is by control of the relative rate of introduction of dopant during deposition, and post formation treatment is by etching with an acid (e.g. oxalic acid) or a base (e.g. NaOH).
U.S. Pat. No. 5,578,502 to Albright et al. describes an improved photovoltaic cell manufacturing process wherein after depositing a film doped with p-type material (e.g. p-type cadmium telluride) the deposited material is subjected to an impurity gettering step in oxygen followed by a recrystallization step in an inert gas.
The paper OPTICAL PROPERTIES OF ALUMINUM DOPED ZINC OXIDE THIN FILMS PREPARED BY RF NAGNETRON SPUTTERING, T. Minami, H. Nanto and S. Takata, Japanese Journal of Applied Physics, Vol. 24, No. 8, August 1985, pp L605-L607, reports average transmittance above 85% for ZnO doped with Al.sub.2 O.sub.3.
The paper by L. A. Harris and R. Schumacher, J Electrochem Soc, 127,1186 (1980) showed hydrogen charging of TiO.sub.2. However, titania treated in this manner is not useful as a transparent conductive metal oxide because the charging effect can be temporary and reversible to an uncharged state.
In spite of the reported advances, the fact remains that achieving 1-2 atomic percent metal reduction is problematic and difficult to control. Hence, there is a need for a method of reliably reducing up to about 2 atomic percent of the metal in a metal oxide.